There are many occasions in telecommunications, optical sensing and other applications when it is necessary to measure the wavelengths of beams of light. A variety of techniques have been devised to accomplish this purpose, but interferometric techniques have proven to be the most practical for wavelength measurement instrumentation. The basis of such techniques is the transformation of optical wavelengths to either a temporal or a spatial fringe frequency that is measured to high accuracy using a scale calibrated against a reference.
A typical Michelson interferometer generates wavelength information from the optical interference of two beams that originate from the same source. The incident beam is split between a fixed path and a varying path. Both beams are reflected back and recombined at the beamsplitter to produce a sinusoidal interference pattern that is a result of the changing phase relationship between the beams. The unknown wavelength of the incident light, λ, can be calculated using the Michelson interferometer equation mλ=2nd. In this equation, m is the number of fringes recorded as the scanning mirror of the Michelson interferometer moves through the distance, d. The refractive index, n, of the medium (typically air) between the mirrors of the interferometer is included to account for the difference between the physical path distance and the optical path distance. The accuracy of this wavelength calculation depends primarily on the precision with which the displacement of the scanning mirror is known. In order to obtain highly accurate wavelength measurements, a reference light source, such as a laser with a stable and accurately known wavelength, is measured simultaneously to determine the scanning mirror displacement in terms of the known wavelength.
Conventional interferometers are capable of producing very precise wavelength measurements. Unfortunately, these conventional interferometers have heretofore not been adaptable for use in portable wavelength measurement devices. This is due to two primary design limitations.
The first design limitation is that the perceived need for a highly stable reference light source has resulted in the use of large and expensive gas lasers as reference sources. Most benchtop wavelength measurement devices use HeNe lasers as a reference source. These lasers are very stable and have a wavelength that is typically two times shorter than the wavelength range used in telecommunication applications. However, HeNe lasers are not suitable for handheld instruments due to their large size and high level of power consumption.
The second design limitation that has prevented the use of interferometers in portable wavelength meters has been the manner in which the scanning mirror is displaced. In a typical Michelson interferometer, such as the interferometer described with reference to U.S. Pat. No. 4,383,762, the scanning mirror is displaced by a mechanical drive which causes the mirror to reciprocate either in translation or in oscillation. This reciprocating movement requires the use of a high precision bearing, which substantially increases the size, cost, and sensitivity of the interferometer to external movement. In addition, the requirement to displace the interferometer's scanning mirror at a substantially constant velocity requires the use of a servo control loop which also contributes to the size, complexity and expense of the interferometer. Further, the reciprocating motion also creates mechanical noise, requires considerable energy and gives rise to large momentum transfers to other instrument components, which must be counteracted in order to avoid measurement instability. Finally, the fact that these interferometers require the use of a fixed mirror located a distance away from the moving mirror increases the size of these interferometers to a point where they are not adapted for use in handheld wavelength meters. Thus, these types of interferometers are only adapted for use in benchtop type wavelength meters.
Another variation of a Michelson interferometer is shown in U.S. Pat. No. 6,124,929. This patent discloses a Michelson interferometer that replaces the reciprocal or translational motion of the moving mirror with a moving mirror that is rotated about a central axis. This arrangement avoids the mechanical noise and momentum transfers inherent in prior interferometers that rely upon a reciprocating motion. However, the device of U.S. Pat. No. 6,124,929 requires the use of a substantially stable reference source, such as a HeNe laser, and the use of a fixed mirror located a distance away from the moving mirror. Accordingly, it is likewise only adapted for use in benchtop type wavelength meters.
A number of handheld wavelength meters have been developed and marketed using non-interferometer based techniques. These meters typically operate using tunable filters. In these meters, the input light is collimated by a lens or concave mirror and then passed through the tunable filter. A photodetector is disposed behind a filter corresponding to a particular wavelength and detects when light of that wavelength passes through the filter. These meters do not require the use of a highly stable reference light or the movable mirrors of an interferometer. Accordingly, they are well adapted for use as portable battery-powered units. Unfortunately, these types of wavelength meters are not well suited to measuring broad spectrum systems for a number of reasons. First, they measure only in the 40 nm range. Moreover, acceptable resolution and accuracy, as required by the wavelength division multiplexing systems commonly used in telecommunications transmissions, are typically in the range of plus or minus 3.3 GHz or plus or minus 25 pm. These types of wavelength meters do not meet these standards. Finally, these types of wavelength meters take up to two minutes to take measurements through the 40 nm range, which is unacceptable in most applications.
Coarse wavelength division multiplexing (CWDM) is a form of wavelength division multiplexing that has wider spacing between wavelengths than dense wavelength division multiplexing (DWDM). CWDM uses a far broader photonic band spectrum than other such systems, which are often confined to one or two channels. Furthermore, up to eighteen wavelengths can be sent using some schemes of CWDM. Modern CWDM and DWDM systems require not only testing to determine if the channel exists and its power level, but also the exact transmitter wavelength, as it is important to make sure the wavelength is within a specified range and is not at the edge of the channel wavelengths. Because filter based meters are essentially “go, no-go” type detectors that will pass wavelengths within the entire channel range, they are not suited to producing the types of measurements that are required in these applications.
Therefore, there is a need for a highly accurate and precise, compact, handheld wavelength meter. In order to determine whether a wavelength is not at an edge of a channel, the precision should be from about a few pm to measuring exact transmission wavelengths. There is also a need for a handheld wavelength meter that can quickly scan, in approximately one second, a full wavelength range of 500 nm (1200-1700 nm) to find all optical channels existing in an optical fiber. There is also a need for a handheld wavelength meter that does not consume a large amount of power during operation and, consequently, provides a long battery life.